Concurrent cracking



Sept. 30, 1969 3,470,263

M. R. KITZEN CONCURRENT CRACKING Filed Feb. 24, 1966 4 Sheets-Sheet 2 PICri-B RU N NUM BER PRODUCTS 1N W1. 70

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p 0, 1969 M. R. KITZEN 3,470,263

CONCURRENT CRACKING Filed Feb. 24, 1966 4 Sheets-Sheet 3 once THROUGH YIELD mom NAPHTHACQTCLHe CONVERSION OF 54%) PRODUCTS !N WT 9 O Q J 8 CW8 3 so "10 6O 50 4.0 30 10 f 10 b s-- C1H 1N TOTAL FEED WT C1 H1 ,NVENTOR 2 A BY Mcxuncz R, Kiqm.

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Sept. 30, 1969 M, Kn'z I 3,470,263

CONCURRENT CRACKING Filed Feb. 24, 1966 4 Sheets-Sheet 4 RAT\O ULTIMATE PRODUCT YIELD TD ETHYLENE YIELD FROM NAPHTHA CZHG m TOTAL FEED wr ULTIMATE YIELD OF C H FROM NAPHTHA 3O SEPARATE CRACKlNCr 80 7O 6O 5O 40 30 20 0 o 1H6 m TOTAL FEED w'rnz, BY mi 1 P r PM AWEYS.

United States Patent 3,470,263 CONCURRENT CRACKING Maurice R. Kitzen, Elkins Park, Pa., assignor to Selas Corporation of America, Dresher, Pa., a corporation of Pennsylvania Continuation-impart of application Ser. No. 411,079,

Nov. 13, 1964. This application Feb. 24, 1966, Ser.

Int. Cl. C07c 3/30, 11/04; C10g 9/100 US. Cl. 260683 Claims ABSTRACT OF THE DISCLOSURE This application is a continuation-in-part of application Ser. No. 411,079, filed Nov. 13, 1964, now Patent No. 3,353,920.

This invention relates to the cracking of petroleum naphtha, and particularly concerns the production of ethylene from petroleum naphtha by cracking.

It is known that naphtha may be cracked under suitable conditions of temperature and pressure in a manner to produce ethylene, and also in a manner to produce ethane. It is further known that ethane is a useful feed material for the production of ethylene by separately cracking the ethane. The cracking of ethane requires considerably more severe cracking conditions than the cracking of petroleum naphtha, particularly higher temperatures.

I have heretofore discovered that considerable advantage can be attained by applying so-called high severity conditions to cracking, in accordance with which the petroleum naphtha is subjected to extreme conditions of temperature and pressure for a very limited period of time, thus cutting the reaction far short of equilibrium conditions, with advantageous results. Such high severity cracking process is disclosed in my copending application Ser. No. 411,079, filed Nov. 13, 1964, the disclosure of which is incorporated herein by reference.

In View of the radically different nature of ethane cracking as distinguished from the cracking of naphtha, it has heretofore been the uniform practice to separate out the ethane that is a product of naphtha cracking and to conduct it to an entirely separate ethane cracking furnace which furnace is operated under extreme high temperature conditions such as 835 C. for example. In such furnace, the ethane is cracked to produce ethylene product and unreacted ethane is recycled for further cracking.

In typical refining operations which produce ethylene, the refinery includes a very considerable number of naphtha crackers which operate at relatively low temperatures, and a very small number of ethane crackers, perhaps only one, specifically arranged for the conversion of ethane to ethylene. The ethane crackers use different tubes, have different tube heights, and require the stocking of a completely separate but full line of spare parts, all of which presents serious disadvantages to the operation of the refinery. Moreover, ethane crackers require quite frequent decoking, on an average of two months for example, which requires a shut-down of the furnace. In contrast, naphtha crackers can run for seven to eight months or longer without requiring decoking.

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It is an object of this invention to provide a means for efiiciently cracking naphtha to produce ethylene, without encountering the disadvantages above referred to.

When naphtha is cracked separately, and when ethane is cracked separately, the total ethylene yield is definite and limited. It is an object of this invention to provide a greater ethylene yield from the same quantities of starting materials, particularly naphtha. Other objects and advantages of this invention will appear in further detail hereinafter and in the drawings of which:

FIG. 1 is a flow diagram showing schematically an arrangement of apparatus for producing ethyene by concurrent cracking in accordance with features of this invention.

FIG. l-A is a chart prepared from actual furnace runs, showing the product distribution in weight percent as ordinate and the percent of ethane in the feed, by Weight percent, as abcissa.

FIGS. 1-B and 1-B1 are charts similar to FIGS. 1-A, showing the product distribution of other products.

FIGS. 2 and 2-A are charts taken from actual furnace runs, showing the once-through yield of certain products from naphtha. In these charts, the ordinate is products in weight percent, and the abcissa is weight percent of ethane in total feed.

FIGS. 3-A and 3-B are charts showing the ratio of ultimate yield of various products from naphtha feed, the ordinates being expressed in terms of Weight percent of product and the abcissa weight percent of ethane in total feed. FIG. 3-A shows various products and FIG. 3-B shows specifically ethylene.

It has been discovered that highly advantageous and unexpected results can be obtained in a method of producing ethylene from petroleum naphtha, by continuously feeding naphtha into a cracking furnace and subjecting it to cracking temperatures which are greater than required to crack naphtha to produce ethylene product and ethane, separating out the ethylene product, and further incorporating additional ethane diluent continuously into the naphtha feed. In each case substantially all the ethane is preferably recycled and thus cracked to extinction. In the concurrent cracking process according to this invention, the cracking temperature is so high as to crack not only the naphtha but the ethane as well.

Thus in accordance with this process, it is unnecessary to maintain and to stock any separate cracking furnace. Moreover, it has been discovered surprisingly that the total ultimate yield of ethylene from the naphtha portion of the feed is significantly greater when the cracking is carried out concurrently with ethane diluent feed, as compared to when naphtha is cracked independently of any ethane diluent in the feed.

It is to be specifically understood that the term ethane diluent refers not only to the ethane that is recycled as a result of its production in the cracking of naphtha, but may refer as well to additional ethane over and above the ethane produced from the cracking of naphtha.

Preferably, in accordance with the method of this invention, the cracking occurs in furnace tubes under high severity conditions of about 600 to 850 C., with a firebox temperature of about 1170-1180 C. and a heat input rate of about 15,000 to 30,000 B.t.u. per hour per square foot of outside tube surface area, with exposure to those conditions of temperature and heat for a period of about 0.1 to 1.5 seconds duration, all as described in detail in my aforesaid copending application. The reaction takes place under a pressure of about 3-4 atmospheres at the crossover point (the point where cracking begins) down to about /2 to 1 /2 atmospheres at the exit, pref- 0 erably.

weight. This total ethane includes recycled ethane from the cracking operation. A highly preferred quantity of ethane in the total ethane plus naphtha feed is about 40% by weight and it has been found that this produces 4 The quantities and results of these eight runs appear in Table l which follows, wherein the input quantities referred to in the table are expressed in metric tons of input per day.

TABLE 1 N o. of Run 1 2 3 4 5 6 7 8' In ut:

Light Naphtha 8.6 16 16 30 40 50 3a Ethane 57 49. 3 44 42 30 20 10 Total 57 58.2 60 5s 60 60 60 63 Steam 24 27. 4 30. 7 30. 5 36 39.8 43. 6 44 Steam/H. o 0. 42 0. 47 0. 51 0. 52 0. 50 0. 66 0. 73 0. 7 Output (wt. percent):

Hz... 3.3 2.2 1.7 1.7 1.4 1.25 1.05 0.9 011a 4. 3 3. 7 10. 6 9. 6 12.1 14. 15. 3 1s. 0,112- 0. 2 0. 2 0. 2 0.13 0. 25 0. 2s 0. 26 0.39 0.11.- 43. a 37. 1 34. 0 s4. 9 33. 1 31. 0 2s. 3 25. 4 0,11. 4a. 3 3s. 4 37. 0 a7. 4 30. 7 23. 9 15.4 5. 6 03H. 1.1 3. 0 4. 6 4. 4 6.9 8.8 10.86 15.0 03115. 0. 07 0. 3 0. 4 0. 46 0. 5 0. 6 0. 62 0. 9 2 04.-.- 2. 2 2. 5 3.05 2 7. 7 118.42 2 C 1. 7 5.3 .1 Con densate 2.2 i 2.1 3.5 5.0 i 13.55

substantially the maximum increase of ethylene ultimate yield over the ultimate yield of ethylene obtamed by separate cracking.

Referring to FIG. 1 of the drawings, the number 10 designates a naphtha cracking furnace having a naphtha feed line 11 and having a separating means such as a still 12 for producing ethylene product through product line 13. Still 12 also produces, through line 14, ethane as a significant product.

The number 15 designates a concurrent cracking furnace having a feed line 16 for naphtha. The products from the concurrent cracking furnace 15 are conducted to a separating device such as a still 17 having a product line 1 8 for ethylene product. Still 17 also has a product line 20 for ethane recycle, which product line 20 is conducted into the naphtha feed line 16. In actual practice, stills 12 and 17 may be one and the same.

An important and essential feature according to this invention is that ethane diluent is added to the feed in the concurrent cracking furnace 15. While this ethane feed may be acquired from any convenient source, it is preferably acquired by means of a conduit 21 running from the ethane product line 14 from the naphtha furnace 10, and connected into the naphtha feed line 16 of the concurrent cracking furnace 15.

After several runs were made with a mixed feed of ethane diluent and light naphtha, results at first did not appear to be encouraging, but as a result of further work it has been discovered that in comparison with separate cracking of naphtha and ethane produced therefrom, an important improvement is attained, particularly in respect to a higher ethylene yield per unit of naphtha input.

TEST RUNS A series of test runs was conducted, these runs being numbered 1 through 8. Run 1 was conducted with only ethane as a feed, including small amounts of hydrogen, methane, ethylene and propylene. Other runs, numbered 2 through 7, were conducted with varying amounts of light naphtha included in the ethane feed, such amounts of naphtha ranging from 15 to 83%. The final run, Run 8, was conducted solely with light naphtha and with no ethane or other diluents added. The total feed compositions are given below:

In each run, the naphtha feed was a light naphtha and the composition of the ethane feed was as follows:

Ethane: Wt. percent H 0.3 CH 0.4 C H 4.2 C H 94.9 C H 0.2

Total 100.0

In each run, thecross-over temperature was approximately 520 C. at a pressure of approximately 3.6 atmospheres, and the outlet temperature was approximately 827 C., at a pressure of 0.9 to 1.0 atmosphere.

FIGS. 1-A, 1-B and l-B-l are graphic representations of the data of Table 1. The dotted lines in these figures represent once-through product yields (obtained by interpolation) which are to be expected for corresponding separate cracking operations. These lines are obtained by proportional interpolation between the points of pure ethane feed on the left and pure naphtha feed on the right.

It is to be noted that up to-about 23% ethane dilution of the feed there is no difference between interpolated yields and actual ethylene yields. However, it is highly significant that at ethane dilutions higher than 23% the test values show much less olefin yield than expected and a much higher yield of ethane plus the equivalent hydrogenated acetylene.

In all comments in connection with this application, where ethane yield is referred to, ethane and hydrogenated acetylene are considered collectively as ethane.

It is further of interest that the propylene yields are lower than would be expected, see the lower part of FIG.

1-B. Further, over a wide range of dilutions the yields of products having five or more carbon atoms are lower. The same is the case with the dry gas yields (methane plus hydrogen)see the upper part of FIG. 1-B.

Referring to the lower right hand corner of FIG. l-A, the line L is drawn directly from zero on the ethane abcissa to on the products ordinate, and thus, if we start from pure naphtha as a feed stock for producing ethylene, the intersection of the line L with the ethane 1, 4, z h, Ce a.

curve which resulted from the actual tests, provides us with a point X which represents the working condition at which the same amount of ethane enters and leaves the furnace. This is at about ethane dilution. Referring again to FIG. 1, it will be appreciated that regardless of the feed stock the ethane is cracked to extinction, and that ethylene can be produced both from naphtha and from ethane.

ETHYLENE YIELD FIGURES percent of ethane in the output divided by the percent ethane in the feed, or (94.9 minus 43.3) divided by 94.9.

It is reasonably to be assumed in all runs that the conversion of the ethane part of the feed remains the same, with the same constant product distribution for all other significant products also. This conversion is, as calculated, 54%. The reason this assumption is proper is that all runs were performed under the same conditions of temperature, pressure and approximate total throughput. This assumption is not meant to represent the actual chemistry that occurs, but is only used as a calculation approach to evaluate the practical consequences of mixed cracking.

Based upon these assumptions, the part of the oncethrough yields of each component which has been contributed both by ethane and naphtha of the feed was calculated. Run 1, as will be evident, gives the product distribution of pure ethane feed.

These calculations are summarized in Table 2 which follows:

TABLE 2.HASS BALANCES (ONCE-THROUGH YIELDS) [All figures in weight percent] Calculated product dis- Input Output Products tribution Reacted Products formed of ethane Light Total C2116 at 54% formed from from light cracked to CgHfl Naphtha Others actual 1 Net conversion reacted ethane naphtha feed extinction Output Products Calculated Reaeted Products formed (once-through) Light Total 02110 at 54% formed from from light yields from (J l-I naphtha Others actual 1 Net conversion reacted 0 H. naphtha feed naphtha feed 2: Hz 0. 26 2. 2, 53 0 5g 3 9 8. 70 3. 31 5.05 33. 6 0.20 0.17 0.03 0. 2 37. 10 33. 37 0. 17 1. 1 38. 40 37. 11 1. 29 8. 6 3. 00 0. 78 2. 05 13. 7 0.30 0. 09 0. 21 1. 4 2. 1. 87 0. 73 4. 9 5. 3O 1. 44 3. 86 25. 7 2. 20 2. 20 14. 7

Total 100. 0 100. 0 100. 0

See footnote at end of table.

TABLE 2.-Contin'ued Input Output Products Calculated Reacted Products formed (once-through) Light Total CzHs at 54% formed from from light yields from CiHs naphtha Others actual 1 Net conversion reacted H. naphtha feed naphtha feed Total 100. 0 100. 0 100. 0

Total 1 Rounded 011 to 100.0.

In the above table it will be appreciated that the column entitled Products formed from reacted C H was obtained using the calculated results of Run 1, taken from the column entitled Products formed from reacted ethane, on the assumption that the same products distribution applies. It will further be appreciated that the column. in Table 2 entitled Calculated (once-through) yields from naphtha feed was calculated on the basis of the applicable factor simply in order to bring the sum of the products up to 100% The results of the calculations given in the last columns of Table 2 are represented in FIGS. 2 and 2-A of the drawings. Also, the data obtained from Run 8, relating to the cracking of light naphtha alone are plotted in FIGS. 2 and 2-A at the abcissa zero. It should be noted that data of Run 1 are not present, as this run was conducted using pure ethane feed. It is significant from the curves in FIGS. 2 and 2-A that up to about 60% C H in the total feed the ethane yield was substantially increased at increasing ethane dilutions of feed, and that this was accompanied by a decrease of propylene yield and a decrease of yield of products having five and more carbon atoms. At ethane dilutions above about the calculated net hydrogen yield for naphtha is negative.

In this region naphtha evidently consumed hydrogen causing an increase in methane and C -pl11s yield. In comparison with the cracking of pure light naphtha, up to 23% ethane dilution there is practically no change in ethylene yield. This agrees well with the ethylene curve of FIG. 1. While at first glance this appears to be disadvantageous, after further study it is clearly demonstrated to be highly advantageous.

Actually, we are not interested in once-through yields of products, because after separation and recycling, the ethane that is produced will be cracked to extinction. Viewed in this light, the above mentioned increasing yields of ethane are most important.

It is now possible to calculate the ultimate yields from naphtha for each component, expressed in weight percent of naphtha input, after recyling ethane, by applying the product distribution given in Table 2, Run 1, of the ethane curve of FIG. 2-A. As we are interested in production of by-products in relation to ethylene production, the ratios of ultimate yields of each component to corresponding ultimate yields of ethylene are also calculated. The results are shown in Table 3, and are graphically represented in FIGS. 3-A and 3-13 of the drawings.

TABLE 3.UL'IIMATE YIELDS FROM NAPHTHA Contributlon (rom produced 0nee- CZHQ (inthrough eluding Ultimate yields hydroyie from genated irom Ratio of Naphtha C111 Naphtha Products Ethylene: 0.766)

Run 2 1. 1 6. 7 7. 8 1 8. 5 14. 7 23. 2 1 12. 5 15. 5 20. 0 1 22. 6 13. 6 36. 2 1 24. 8 10. 9 35. 7 1 25. 6 7. 7 33. 0 1 25. 4 4. 6 30. 0 1 (i= 0) 0) t) See footnote at end of table.

TABLE 3.Continued Contribution from produced Once- 02115 (inthrough eluding Ultimate yields hydroyield from genated from Ratio of Naphtha CzH2 N aphtha Products Dry Gas (CH4+H2)I (f= 0.134)

Run 2 29. 7 1. 2 30.9 3. 96 25. 2 2. 6 27.8 1. 20 21. 1 2. 9 24. 0. 83 19. 4 2. 4 21. 8 0.59 19.2 1. 9 21. 1 0. 59 18. 2 1. 0 19.2 0. 58 19. 4 0. 8 20. 2 0. 67

1 Once-through 0 1-10 yield from naphtha (-I-CzHz) X f.

2 0 for all runs.

As will be observed, the result as shown in FIG. 3-B shows a very significant increase of ultimate yield of ethylene from naphtha, as compared to the ultimate yield otbained by separate cracking. Apparently the presence of ethane together with the presence of naphtha results in a shift of reactions within the cracking furnace, catalyzing the further production of ethylene-a phenomenon not heretofore discovered to the best of my knowledge.

The maximum ethylene yield, as shown in FIG. 3-B, at about 40% ethane dilution is about 36.5% which represents an increase of 6.5% (absolute) in comparison with separate naphtha cracking. The increased ultimate yield of ethylene from naphtha is probably caused by an increased once-through yield of ethane from naphtha, although this is not absolutely certain. At the same time yields of by-products tend to show a reduction. The reduction of dry gas is particularly important, since it affects substantially the operating costs for compression.

In common practice, in refineries practicing the cracking of petroleum naphtha, no outside ethane source will be available. As already pointed out, in that case the furnace should preferably be run (for the specific light naphtha that was used in Runs 1-8) at approximately 15% ethane dilution which will thus provide an increase of ultimate ethylene yield from 30.0 to 33.3%. As the entering ethane stream equals the outgoing one, the production of all components can be regarded as contributed by naphtha feed only. Only at 15 ethane dilution, multiplication by a factor of 100/85 of the values of FIG. 2 results directly in the corresponding points in FIG. 3, for that naphtha.

It will be understood, however, that it is an advantage of this invention that flexibility of choice is offered as to the percent ethane diluent in the feed, and that it is not necessary to adhere to the 15% figure above referred to.

For example, in a plant having a plurality of furnaces, some could be supplied with relatively low quantities of ethane diluent (or none) and others with relatively large quantities, in order to attain the maximum ethylene yield under the actual operating conditions. Indeed, one or more furnaces may then be provided with the optimum amount of ethane diluent (about 40-50% in FIG. 3-B) to produce the optimum ultimate ethylene yield (36 .2% in Table 3, Run 5). In other cases, balances can be made and the furnaces otherwise controlled to obtain the best total productivity. The foregoing comments apply when the composition of all the naphtha feed is the same for all the furnaces, or when diflerent naphtha feeds are used for some furnaces.

Wholly aside from the fact that concurrent cracking gives higher ultimate ethylene yields, it will be appreciated that another practical consequence is the possibility of avoiding the installation of a separate ethane furnace, and of maintaining separate replacement and repair parts, stocks, etc.

Although this invention has been disclosed with respect to specific tests performed on specific products it will be appreciated that it is adaptable to various forms of naphtha starting materials and that variations may be made in temperatures and pressures in order to produce optimum results with particular naphtha feeds. Also it will be appreciated that various equivalents may be substituted for those specifically referred to, all without departing from the spirit and scope of the invention as defined in the appended claims.

The following is claimed:

1. In the method of producing ethylene from naphtha, the steps which comprise cracking naphtha separately under high severity conditions of at least about 600 C. to produce ethane and ethylene product, and separately concurrently crackling naphtha with ethane feed under high severity conditions of at least about 600 C., which ethane feed is said ethane product from the naphtha process first mentioned.

2. The method defined in claim 1 wherein the concurrent cracking operation produces ethylene product and further ethane, and wherein said further ethane is recycled to extinction.

3. The method of any one of claims 1 and 2 wherein the cracking occurs under a heat input rate of about 15,000 to 30,000 Btu. per hour per square foot of outside tube surface area, and an exposure for a period of about 0.1 to 1.5 seconds. i

4. The invention of any one of claims 1 and 2 wherein the amount of ethane in the total ethane plus naphtha feed is in the range of about 10-60% by weight.

5. The invention of any one of claims 1 and 2 wherein the amount of ethane in the total ethane plus naphtha feed is about 40% by weight and produces substantially the maximum increase of ethylene ultimate yield over the ultimate yield of ethylene attained by separate cracking.

References Cited UNITED STATES PATENTS 2,301,548 11/1942 Koch 208-72 2,904,502 9/1959 Shapleigh 260683 2,917,564 12/1959 Pollock 260-683 FOREIGN PATENTS 474,635 6/1951 Canada. 516,814 9/1955 Canada.

DELBERT E. GANTZ, Primary Examiner C. E. SPRESSER, Assistant Examiner US. Cl. X.R. 20877, 128 

